1. Field of the Invention
This invention relates generally to measurement and data acquisition systems and, more particularly, to a circuit for protecting general-purpose digital input/output lines against destructive electrical conditions.
2. Description of the Related Art
Scientists and engineers often use measurement systems to perform a variety of functions, including measurement of a physical phenomena or unit under test (UUT), test and analysis of physical phenomena, process monitoring and control, control of mechanical or electrical machinery, data logging, laboratory research, and analytical chemistry, to name a few examples.
A typical measurement system comprises a computer system with a measurement device or measurement hardware. The measurement device may be a computer-based instrument, a data acquisition device or board, a programmable logic device (PLD), an actuator, or other type of device for acquiring or generating data. The measurement device may be a card or board plugged into one of the I/O slots of the computer system, or a card or board plugged into a chassis, or an external device. For example, in a common measurement system configuration, the measurement hardware is coupled to the computer system through a PCI bus, PXI (PCI extensions for Instrumentation) bus, a GPIB (General-Purpose Interface Bus), a VXI (VME extensions for Instrumentation) bus, a serial port, parallel port, or Ethernet port of the computer system. Optionally, the measurement system includes signal conditioning devices which receive field signals and condition the signals to be acquired.
A measurement system may typically include transducers, sensors, or other detecting means for providing “field” electrical signals representing a process, physical phenomena, equipment being monitored or measured, etc. The field signals are provided to the measurement hardware. In addition, a measurement system may also typically include actuators for generating output signals for stimulating a UUT.
Measurement systems, which may also be generally referred to as data acquisition systems, may include the process of converting a physical phenomenon (such as temperature or pressure) into an electrical signal and measuring the signal in order to extract information. PC-based measurement and data acquisition (DAQ) systems and plug-in boards are used in a wide range of applications in the laboratory, in the field, and on the manufacturing plant floor, among others.
In a measurement or data acquisition process, analog signals may be received by a digitizer, which may reside in a DAQ device or instrumentation device. The analog signals may be received from a sensor, converted to digital data (possibly after being conditioned) by an Analog-to-Digital Converter (ADC), and transmitted to a computer system for storage and/or analysis. When a measurement system generates an output analog signal, the computer system may generate digital signals that are provided to one or more digital to analog converters (DACs) in the DAQ device. The DACs may convert the digital signal to an output analog signal that is used, e.g., to stimulate a UUT.
Multifunction DAQ devices typically include digital I/O capabilities in addition to the analog capabilities described above. Digital I/O applications may include monitoring and control applications, video testing, chip verification, and pattern recognition, among others. DAQ devices may include one or more general-purpose, bidirectional digital I/O lines to transmit and receive digital signals to implement one or more digital I/O applications.
General-purpose, bidirectional digital I/O lines are typically susceptible to destructive electrical conditions, such as over-voltage, over-current, and electrostatic discharge (ESD). A common solution to protect against over-voltage and/or ESD in electronic devices is to include one or more voltage clamping devices external to the device and on the lines to be protected to prevent damage from these electrical conditions. When the voltage rises above the voltage limit for the device, the voltage clamping devices may protect the device from over-voltage conditions by clamping or fixing the voltage at approximately the voltage limit. The voltage clamping device preferably sinks any excess current to block the current from damaging the device. In some implementations, two clamping diodes external to the device may be used for clamping the signal to the positive voltage rail and ground of the device. The diodes may keep the voltage at the protected node in check, as long as they can withstand the high currents that occur when the diodes starts clamping. Therefore, in some implementations, a clamping circuit such as this one may convert the problem of over-voltage into a problem of over-current.
Diodes typically have a limit on the peak current and the sustained current they can withstand. The sustained or continuous current limit is the maximum current the diodes are guaranteed to withstand for an undefined period of time without damage. Above this limit, the diode may only withstand the current for a limited period of time, i.e., the larger the current, the shorter the period of time. In the case of the circuit including two diodes, they may only offer protection against very short duration over-voltage conditions before the diodes themselves suffer damage. The current through the diodes may only be limited by the small impedance of the diodes themselves and the impedance of the voltage source associated with the voltage rails of the device. The I-V curve of a diode is an exponential function, and it typically does not take too much over-voltage to exceed the maximum continuous current of the diode.
It is an industry standard to protect all input and general-purpose, bidirectional I/O terminals in a semiconductor device with internal clamping diodes similar to the external clamping diodes described above. When the internal diodes are voltage clamping, there may be the added risk of activating a latchup condition which is a potentially self destructing mode the semiconductor can get into where the circuitry draws excess current, and may only get out of it by removing power to the semiconductor. The internal clamping diodes are typically designed to clamp over-voltage or ESD events which are very short on duration.
In other implementations, a resistor in series with the line to be protected may be added to the protection circuit described above to control the current once the diodes enter a clamping mode. The resistor may limit the current through the diodes when they are voltage clamping, which typically extends the over-voltage protection range, because it takes a greater over-voltage to reach the maximum continuous current of the diodes. However, at the same time the resistor may increase the line impedance in normal operation, which may reduce the normal current possible for a given output voltage. In sum, the higher the resistance of the resistor, the higher the over-voltage protection range, but typically the lower the performance of the device.
Furthermore, in the protection circuit described above, which includes the resistor in series with the I/O line, and the internal and external clamping diodes, there may also be a tradeoff between the power dissipation of the components and the level of protection. The equation for the power dissipation on the resistor is proportional to the square of the over-voltage and is inversely proportional to the resistor value. Power dissipation typically becomes an important issue since it is desirable to offer the greatest over-voltage protection while having the resistance of the added resistor to be as small as possible.